In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic printing plates typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. The imageable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working. Conversely, if the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital or thermal imaging has become increasingly important in the printing industry because of their stability to ambient light. The imageable elements used for the preparation of lithographic printing plates (that is, lithographic printing plate precursors) have been designed to be sensitive to heat or infrared radiation and can be exposed using thermal heads of more usually, UV, visible, or infrared laser diodes that image in response to signals from a digital copy of the image in a computer a platesetter. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the elements.
Various radiation-sensitive compositions are used in negative-working lithographic printing plate precursors as described for example in U.S. Pat. No. 6,309,792 (Hauck et al.), U.S. Pat. No. 6,893,797 (Munnelly et al.), U.S. Pat. No. 6,727,281 (Tao et al.), U.S. Pat. No. 6,899,994 (Huang et al.), and U.S. Pat. No. 7,429,445 (Munnelly et al.), U.S. Patent Application Publications 2002/0168494 (Nagata et al.), 2003/0118939 (West et al.), and EP Publications 1,079,276A2 (Lifka et al.) and 1,449,650A2 (Goto et al.).
After imaging, the imaged precursors are developed (processed) to remove the non-imaged (non-exposed) regions of the imageable layer. Often, lithographic printing plate precursors are designed with a water-soluble topcoat or water-soluble oxygen-impermeable barrier layer disposed over the negative-working, photosensitive imageable layer. This topcoat is used to improve high polymerization rate during imaging by assuring higher imageable layer sensitivity. Such water-soluble topcoats can be removed prior to or during processing of the imaged precursor.
The imaged precursors can be processed (developed) off-press by using a suitable processing solution (developer) that removes the non-exposed regions of the imageable layer. Alternatively, the imaged precursors can be processed on-press (without intermediate processing) at the beginning of the printing operation.
Such lithographic printing plate precursors containing a water-soluble topcoat are generally transported and stored after manufacture in stacks of dozens or hundreds of individual precursors. Interleaf paper is often inserted between individual adjacent precursors to prevent scratches in the topmost surface. However, despite the presence of interleaf papers, the water-soluble topcoat surface can still be scratched during transportation or handling (for example, when removing the interleaf paper in an automated plate loader), leading to a loss in sensitivity in the scratched areas. Furthermore, there is a need for improving the reliability of separating adjacent lithographic printing plate precursors and interleaf papers, especially when automized plate loaders are used.
U.S. Patent Application Publication 2010/0151385 (Ray et al.) describes negative-working lithographic printing plate precursors supplied in a stack. Each precursor has a water-soluble topcoat having a dry coating weight of less than 1 g/m2 so that interleaf paper can be omitted from the stack. Large organic or inorganic particles (1-6 μm) are optional relative to the topcoat thickness or dry coating weight (<1 g/m2).
U.S. Pat. No. 5,563,023 (Kangas et al.) describes photoimageable elements having a protective overcoats can contain organic polymeric beads to provide anti-blocking properties, which organic polymeric beads have a particle size range of 0.75-75 μm.
U.S. Patent Application Publication 2011/0053085 (Huang et al.) describes lithographic printing plate precursors that are designed to be stored, shipped, and used in stacks without interleaf paper between individual precursors. This result is achieved by incorporating certain core-shell polymeric particles having an average diameter of from 3 μm to 20 μm into the outermost precursor layer such as an oxygen barrier topcoat.
In addition, U.S. Pat. No. 8,105,751 (Endo) describes the use of silica-coated organic resin fine particles in mica-containing uppermost protective layers of lithographic printing plate precursors.
It was found in extensive studies that the blocking properties between lithographic printing plate precursors in a stack without interleaf paper can be improved with particles having particles sizes of at least 1.5 times of the oxygen barrier topcoat thickness. Furthermore, it was found that the reliability of separating interleaf paper and plates is improved with those particles.
However, the disadvantage of this approach is that the susceptibility for scratching during transportation and handling is increased.
It was recently found that improvements in outer layer scratch resistance can be obtained by incorporating hydrocarbon wax particles in such layers, as described in copending and commonly assigned U.S. Ser. No. 13/482,151 (filed May 29, 2012 by Balbinot and Jarek).
There is a need for further improvement in scratch resistance and a lower tendency for “blocking” during stacking, transportation, and use of negative-working lithographic printing plate precursors. It is also desirable to have the option to avoid the use of interleaf papers without a reduction in scratch resistance or an increase in the “blocking” tendency in the stacked precursors.